Process for separating components of a polymer-monomer mixture obtained by high-pressure polymerization of ethylenically unsaturated monomers

ABSTRACT

A process for separating polymeric and gaseous components of a polymer-monomer mixture at a pressure of from 0.12 MPa to 0.6 MPa and a temperature of from 120° C. to 300° C. in a separation vessel is provided. The separation vessel has a vertically arranged cylindrical shape with a ratio of length to diameter L/D of from 0.6 to 10 and an inlet pipe capable of introducing the polymer-monomer mixture into the separation vessel which the inlet pipe extends vertically from the top of the separation vessel into the separation vessel. Further a process for preparing ethylene homopolymers or copolymers from ethylenically unsaturated monomers in the presence of free-radical polymerization initiators at temperatures from 100° C. to 350° C. and pressures in the range of from 110 MPa to 500 MPa comprising such a process for separating a polymer-monomer mixture is provided.

FIELD OF THE INVENTION

The present disclosure provides a process for separating a mixturecomprising polymeric components and gaseous components into a liquidfraction and a gaseous fraction, where the polymeric components of themixture are obtained by high-pressure polymerization of ethylenicallyunsaturated monomers in the presence of at least one free-radicalpolymerization initiator. In some embodiments, the present disclosureprovides a process for preparing ethylene homopolymers or copolymersfrom ethylenically unsaturated monomers in the presence of free-radicalpolymerization initiators.

BACKGROUND OF THE INVENTION

Polyethylene is the most widely used commercial polymer. It can beprepared by a couple of different processes. Polymerization in thepresence of free-radical initiators at elevated pressures was the methodfirst discovered to obtain polyethylene and continues to be a valuedprocess with high commercial relevance for the preparation of lowdensity polyethylene (LDPE).

A normal set-up of a plant for preparing low density polyethylenecomprises, a polymerization reactor which can be an autoclave or atubular reactor or a combination of such reactors and additionalequipment. For pressurizing the reaction components, usually a set oftwo compressors, a primary and a secondary compressor, is used. At theend of the polymerization sequence, a high-pressure polymerization unitthat may include apparatuses like extruders and granulators forpelletizing the resulting polymer can be used. Furthermore, such apolymerization unit generally also comprises means for feeding monomersand comonomers, free-radical initiators, modifiers or other substancesat one or more positions to the polymerization reaction.

A characteristic of the radically initiated polymerization ofethylenically unsaturated monomers under high pressure is that theconversion of the monomers is generally not complete. For each pass ofthe reactor, only about 10% to 50% of the dosed monomers are convertedin the case of polymerization in a tubular reactor, and from 8% to 30%of the dosed monomers are converted in the case of polymerization in anautoclave reactor. Accordingly, it is common practice to separate thedischarged reaction mixture into polymeric and gaseous components andrecycle the monomers. To avoid unnecessary decompression and compressionsteps, the separation into polymeric and gaseous components is usuallycarried out in two stages. The reaction mixture leaving the reactor istransferred to a first separating vessel, frequently called ahigh-pressure product separator, in which the separation in polymericand gaseous components is carried out at a pressure that allows forrecycling of the ethylene and comonomers separated from the reactionmixture to the reaction mixture at a position between the primarycompressor and the secondary compressor. At the conditions for operatingthe first separation vessel, the polymeric components within theseparating vessel are in a liquid state. The liquid phase obtained inthe first separating vessel is transferred to a second separationvessel, frequently called a low-pressure product separator, in which afurther separation into polymeric and gaseous components takes place atlower pressure. The ethylene and additional comonomers separated fromthe mixture in the second separation vessel are fed to the primarycompressor, where they are compressed to the pressure of the freshethylene feed, combined with the fresh ethylene feed and the joinedstreams are further pressurized to the pressure of the high-pressure gasrecycle stream. The level of the liquid phases in the first and thesecond separating vessels are generally measured by radiometric levelmeasurement and are controlled automatically by product dischargevalves.

Separating polymeric and gaseous components of a reaction mixtureobtained by high-pressure polymerization of ethylenically unsaturatedmonomers in the presence of free-radical polymerization initiators bylowering pressure and temperature has already been described in GB580182. Apparatuses of different design and geometry have been disclosedas low pressure separating vessels operating in a pressure range of from0.12 MPa to 0.6 MPa. DE 1219378 describes low pressure separators, inwhich polyethylene is homogenized by intensive stirring. DD 1219378refers to a two-stage product separation process, in which an lowpressure separator is employed into which the polymer melt is introducedat a point which is at 60% to 100% of the total height of the lowpressure separator, through a pipe which is directed downwards at anangle of 50° to 70° to the horizontal close to the wall of the vessel.Preferably the melt inlet is at 60% to 80% of the total height of thelow pressure separator. WO 2011/078856 A1 discloses a low pressureseparation vessel which includes a cylindrical section that ends in afrustroconical section at its bottom and which has a product inletmounted on its side for receiving the polymer product from the highpressure separator.

Processes for separating polymeric and gaseous components of acomposition obtained by high-pressure polymerization of ethylenicallyunsaturated monomers allow for the recycling of ethylene, comonomers andother low-molecular weight components from the reaction mixture to thesuction side of the secondary compressor. However, there are still aconsiderable amount of polymeric components carried over by the gasstream leaving the second separating vessel. These components have to beseparated from the gas in subsequent separation steps in the ethylenerecycle line. Furthermore, entrained polymeric components can adhere tothe surface of the separation vessel or to the surface of transferconduits and cause fouling.

Accordingly there is a need in the art to overcome the disadvantages ofthe prior art and to provide separation processes which show a very lowamount of polymer carry over with the gas stream leaving the secondseparation vessel. Furthermore, there is a need in the art for processesthat allow longer periods of production without the necessity ofcleaning the separation vessel and a fast grade change between thedifferent types of produced, low density polyethylenes with a reducedamount of out-of-specification polymer. In addition, since theseparating vessels in high-pressure processes for preparing olefinpolymers in the presence of free-radical polymerization initiators arelarge, pressure-resistant apparatuses, there is a constant desire foreconomic reasons to be able to construct such separation vessels assmall as possible without losing a good separating efficiency.

SUMMARY OF THE INVENTION

The present disclosure provides a process for separating apolymer-monomer mixture comprising polymeric components and gaseouscomponents into a liquid fraction and a gaseous fraction, the processcomprising the steps of:

(i) introducing the polymer-monomer mixture into a separation vessel;(ii) separating the polymer-monomer mixture at a pressure of from 0.12MPa to 0.6 MPa and a temperature of from 120° C. to 300° C. into agaseous fraction and a liquid fraction;(iii) withdrawing the gaseous fraction from the top of the separationvessel; and(iv) withdrawing the liquid fraction from the bottom of the separationvessel,wherein the polymeric components of the mixture are obtained byhigh-pressure polymerization of ethylenically unsaturated monomers inthe presence of at least one free-radical polymerization initiator,and whereina) the separation vessel has a vertically arranged cylindrical shapewith a ratio of length to diameter L/D of from 0.6 to 10;b) the separation vessel is equipped with an inlet pipe capable ofintroducing the polymer-monomer mixture into the separation vessel;c) the inlet pipe extends vertically from the top of the separationvessel into the separation vessel; andd) the ratio of the inner diameter of the inlet pipe at its lower endand the inner diameter of the separating vessel in its cylindrical partis in the range of from 0.15 to 0.4.

In some embodiments, the inlet pipe is centrally arranged in theseparation vessel.

In some embodiments, the polymer-monomer mixture has been obtained asliquid fraction separating a reaction mixture obtained by high-pressurepolymerization and the separation was carried out at a pressure of from15 MPa to 50 MPa and a temperature of from 120° C. to 300° C.

In some embodiments, the inlet pipe extends for a distance into theseparation vessel which is from 25% to 40% of the length of theseparation vessel.

In some embodiments, wherein the velocity of the polymer-monomer mixtureat the lower end of the inlet pipe is in the range of from 0.3 m/s to 20m/s.

In some embodiments, the vertical velocity of the rising gas in theregion from the lower end of the inlet pipe to the outlet forwithdrawing the gaseous fraction from the separation vessel is in therange of from 0.02 m/s to 2.0 m/s.

In some embodiments, the surfaces within the separation vessel which arein contact with the liquid fraction have an average roughness Ra of atthe most 3.2 μm.

In some embodiments, the surfaces within the separation vessel which arein contact with the liquid fraction are provided with apolytetrafluoroethylene coating.

In some embodiments, the angle of the conical part of the separator isin the range of from 30 to 60°.

In some embodiments, inlet pipe has a conical part which has an angle inthe range of from 5 to 25°.

In some embodiments, the level of the liquid fraction in the separationvessel is measured by radiometric level measurement and is controlled bya product discharge valve.

In some embodiments, the conduit providing the polymer-monomer mixtureto the separation vessel has a bend in the range of from 60° to 120°.

In some embodiments, the separation vessel is mounted to a pelletizingdevice.

In some embodiments, the present disclosure provides a process forpreparing ethylene homopolymers or copolymers comprising polymerizingethylenically unsaturated monomers in the presence of free-radicalpolymerization initiators at temperatures from 100° C. to 350° C. andpressures in the range of from 110 MPa to 500 MPa in a polymerizationreactor, the process comprising a process for separating polymeric andgaseous components as described above.

In some embodiments, the polymerization is carried out in one or moretubular reactors or autoclave reactors or combinations of such reactors

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a set-up for a suitable tubularpolymerization reactor, without restricting the disclosure to theembodiments described therein.

FIG. 2 shows schematically a cross-section of a separation vesselsuitable for the separating process of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present disclosure refers to separating apolymer-monomer mixture comprising polymeric components and gaseouscomponents into a liquid fraction and a gaseous fraction, where thepolymeric components of the mixture are obtained by high-pressurepolymerization of ethylenically unsaturated monomers in the presence ofat least one free-radical polymerization initiator. In an embodiment,the process of the present disclosure refers to separating apolymer-monomer mixture which was obtained as liquid fraction in thefirst separation vessel after a high-pressure polymerization.

In some embodiments, the high-pressure polymerization is ahomopolymerization of ethylene or a copolymerization of ethylene withone or more other monomers, provided that these monomers arefree-radically copolymerizable with ethylene under high pressure.Examples of suitable copolymerizable monomers are α,β-unsaturatedC₃-C₈-carboxylic acids, including maleic acid, fumaric acid, itaconicacid, acrylic acid, methacrylic acid and crotonic acid, derivatives ofα,β-unsaturated C₃-C₈-carboxylic acids, e.g. unsaturatedC₃-C₁₅-carboxylic esters, including esters of C₁-C₆-alkanols, oranhydrides such as methyl methacrylate, ethyl methacrylate, n-butylmethacrylate, tert-butyl methacrylate, methyl acrylate, ethyl acrylate,n-butyl acrylate, 2-ethylhexyl acrylate, tert-butyl acrylate,methacrylic anhydride, maleic anhydride or itaconic anhydride, and1-olefins such as propene, 1-butene, 1-pentene, 1-hexene, 1-octene or1-decene. In addition, vinyl carboxylates such as vinyl acetate can beused as comonomers. In some embodiments, propene, 1-butene, 1-hexene,acrylic acid, n-butyl acrylate, tert-butyl acrylate, 2-ethylhexylacrylate, vinyl acetate or vinyl propionate may be used as a comonomer.

In the case of copolymerization, the proportion of comonomer orcomonomers in the reaction mixture is from 1 to 50% by weight, such asfrom 3 to 40% by weight, based on the amount of monomers, i.e. the sumof ethylene and other monomers. Depending on the type of comonomer, thecomonomers may be fed at more than one point to the reactor set-up. Insome embodiments, the comonomers are fed to the suction side of thesecondary compressor.

For the purposes of the present disclosure, polymers or polymericmaterials are all substances which are made up of at least two monomerunits, including low density polyethylenes having an average molecularweight M_(n) of more than 20 000 g/mole. The term “low densitypolyethylene” is meant to include ethylene homopolymers and ethylenecopolymers. The process of the present disclosure may also be employedin the preparation of oligomers, waxes and polymers having a molecularweight M_(n) of less than 20 000 g/mole.

Possible initiators for starting the free-radical polymerization in therespective reaction zones are in general all substances that can produceradical species under the conditions in the polymerization reactor, forexample, oxygen, air, azo compounds and peroxidic polymerizationinitiators. In one embodiment, the polymerization is carried out usingoxygen, either fed in the form of pure O₂ or as air. In case ofinitiating the polymerization with oxygen, the initiator is normallyfirst mixed with the ethylene feed and then fed to the reactor. In sucha case it is not only possible to feed a stream comprising monomer andoxygen to the beginning of the polymerization reactor but also to one ormore points along the reactor creating two or more reaction zones.Initiation using organic peroxides or azo compounds also represents anembodiment of the present disclosure. Examples of suitable organicperoxides are peroxy esters, peroxy ketals, peroxy ketones andperoxycarbonates, e.g. di(2-ethylhexyl) peroxydicarbonate, dicyclohexylperoxydicarbonate, diacetyl peroxydicarbonate, tert-butylperoxyisopropylcarbonate, di-sec-butyl peroxydicarbonate, di-tert-butylperoxide, di-tert-amyl peroxide, dicumyl peroxide,2,5-dimethyl-2,5-di-tert-butylperoxyhexane, tert-butyl cumyl peroxide,2,5-dimethyl-2,5-di(tert-butylperoxy)hex-3-yne, 1,3-diisopropylmonohydroperoxide or tert-butyl hydroperoxide, didecanoyl peroxide,2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, tertamylperoxy-2-ethylhexanoate, dibenzoyl peroxide, tert-butylperoxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, tert-butylperoxydiethylisobutyrate, tert-butyl peroxy-3,5,5-trimethylhexanoate,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,1,1di(tert-butylperoxy)cyclohexane, tert-butyl peroxyacetate, cumylperoxyneodecanoate, tert-amyl peroxyneodecanoate, tert-amylperoxypivalate, tert-butyl peroxyneodecanoate, tert-butyl permaleate,tert-butyl peroxypivalate, tert-butyl peroxyisononanoate,diisopropylbenzene hydroperoxide, cumene hydroperoxide, tert-butylperoxybenzoate, methyl isobutyl ketone hydroperoxide,3,6,9-triethyl-3,6,9-trimethyl-triperoxocyclononane and2,2-di(tert-butylperoxy)butane. Azoalkanes (diazenes), azodicarboxylicesters, azodicarboxylic dinitriles such as azobisisobutyronitrile andhydrocarbons which decompose into free radicals and are also referred asC—C initiators, e.g. 1,2-diphenyl-1,2-dimethylethane derivatives and1,1,2,2-tetramethylethane derivatives, are also suitable. It is possibleto use either individual initiators or mixtures of various initiators. Alarge range of initiators, such as peroxides, are commerciallyavailable, for example the products of Akzo Nobel offered under thetrade names Trigonox® or Perkadox®.

Suitable peroxidic polymerization initiators include, for example,1,1-di(tert-butylperoxy)cyclohexane, 2,2-di(tert-butylperoxy)butane,tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl peroxybenzoate,2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl cumyl peroxide,di-tert-butyl peroxide and2,5-dimethyl-2,5-di(tert-butylperoxy)hex-3-yne. In an embodiment, theperoxidic polymerization initiators are tert-butylperoxy-3,5,5-trimethylhexanoate, di-(2-ethylhexyl)peroxydicarbonate ortert-butyl peroxy-2-ethylhexanoate.

The initiators can be employed individually or as a mixture inconcentrations of from 0.1 mol/t to 50 mol/t of polyethylene produced,including from 0.2 mol/t to 20 mol/t, in each reaction zone. In oneembodiment, the free-radical polymerization initiator, which is fed to areaction zone, is a mixture of at least two different azo compounds ororganic peroxides. If such initiator mixtures are used, the initiatormixtures may be fed to all reaction zones. There is no limit for thenumber of different initiators in such a mixture, however in someembodiments the mixtures are composed of from two to six, including two,three or four different initiators. In certain embodiments, mixtures ofinitiators which have different decomposition temperatures are used.

In some embodiments, the initiators are used in a dissolved state.Examples of suitable solvents are ketones and aliphatic hydrocarbons,including octane, decane and isododecane, and other saturatedC₈-C₂₅-hydrocarbons. The solutions may comprise the initiators orinitiator mixtures in proportions of from 2 to 65% by weight, such asfrom 5 to 40% by weight and from 8 to 30% by weight.

In the high-pressure polymerization process of the present disclosure,the molecular weight of the polymers to be prepared can be altered bythe addition of modifiers, which act as chain-transfers agents. Examplesof suitable modifiers are hydrogen, aliphatic and olefinic hydrocarbons,e.g. propane, butane, pentane, hexane, cyclohexane, propene, 1-butene,1-pentene, and 1-hexene, ketones such as acetone, methyl ethyl ketone(2-butanone), methyl isobutyl ketone, methyl isoamyl ketone, diethylketone and diamyl ketone, aldehydes such as formaldehyde, acetaldehydeand propionaldehyde, and saturated aliphatic alcohols such as methanol,ethanol, propanol, isopropanol and butanol. In certain embodiments,saturated aliphatic aldehydes such as propionaldehyde or 1-olefins suchas propene, 1-butene and 1-hexene, or aliphatic hydrocarbons such aspropane may be used.

In some embodiments, the high-pressure polymerization is carried out atpressures of from 110 MPa to 500 MPa, including from 160 MPa to 350 MPaand from 200 MPa to 330 MPa for polymerization in a tubular reactor, andpressures of from 110 MPa to 300 MPa and from 120 MPa to 280 MPa forpolymerization in an autoclave reactor. In certain embodiments, thepolymerization temperatures are in a range of from 100° C. to 350° C.,from 180° C. to 340° C. and from 200° C. to 330° C. for polymerizationin a tubular reactor, and in a range of from 110° C. to 320° C. and from120° C. to 310° C. for polymerization in an autoclave reactor.

The polymerization can be carried out with all types of high-pressurereactors appropriate for high-pressure polymerization. Suitablehigh-pressure reactors are, for example, tubular reactors or autoclavereactors or combinations of such reactors. In some embodiments, thehigh-pressure reactors are tubular reactors or autoclave reactors.

In certain embodiments, high-pressure autoclave reactors for use in thepresent technology include stirred reactors having a length-to-diameterratio of in the range from 2 to 30, such as from 2 to 20. Such autoclavereactors may have one or more reaction zones, including from 1 to 6reaction zones and from 1 to 4 reaction zones. The number of reactionzones depends from the number of agitator baffles, which separateindividual mixed zones within the autoclave reactor.

Appropriate tubular reactors for use in the present technology are long,thick-walled pipes, which in some embodiments are about 0.5 km to 4 km,including from 1 km to 3 km and from 1.5 km to 2.5 km, in length. Insome embodiments, the inner diameter of the pipes is usually in therange of from about 30 mm to 120 mm, including from 60 mm to 100 mm.Such tubular reactors may have a length-to-diameter ratio of greaterthan 1000, including from 10000 to 40000 and from 25000 to 35000.

In further embodiments, tubular reactors for use in the presentdisclosure have at least two reaction zones, such as from 2 to 6reaction zones and from 2 to 5 reaction zones. The number of reactionzones is given by the number of feeding points for the initiator. Such afeeding point can, for example, be an injection point for a solution ofazo compounds or organic peroxides. Fresh initiator may be added to thereactor, where it decomposes into free radicals and initiates furtherpolymerization. The heat generated by the reaction increases thetemperature of the reaction mixture, as more heat is generated than canbe removed through the walls of the tubular reactor. The risingtemperature increases the rate of decomposition of the free-radicalinitiators and accelerates polymerization until essentially all of thefree-radical initiator is consumed. No further heat is generated and thetemperature decreases again since the temperature of the reactor wallsis lower than the temperature of the reaction mixture. Accordingly, insome embodiments the part of the tubular reactor downstream of aninitiator feeding point in which the temperature rises is the reactionzone, while the part thereafter, in which the temperature decreasesagain, is predominantly a cooling zone. The amount and nature of addedfree-radical initiators determines how much the temperature rises andaccordingly allows for adjusting that value. In certain embodiments, thetemperature rise is in a range of from 70° C. to 170° C. in the firstreaction zone and 50° C. to 130° C. for the subsequent reaction zones,depending on the product specifications and the reactor configuration.

The compression of the reaction gas composition to the polymerizationpressure may be carried out, in some embodiments, by at least twosequentially operating compressors, of which a primary compressor firstcompresses the reaction gas composition to a pressure of from 10 MPa to50 MPa and a secondary compressor, which is sometimes designated ashyper compressor, further compresses the reaction gas composition to thepolymerization pressure of from 110 MPa to 500 MPa. In certainembodiments, the primary compressor and the secondary compressor aremultistage compressors. It is further possible to separate one or morestages of one or both of these compressors and divide the stages intoseparated compressors. However, in further embodiments a series of oneprimary compressor and one secondary compressor is used for compressingthe reaction gas composition to the polymerization pressure. In suchcases, sometimes the whole primary compressor is designated as theprimary compressor. However, it is also common to designate the one ormore first stages of the primary compressor, which compress the recyclegas from the low-pressure product separator to the pressure of the freshethylene feed, as the booster compressor, and then only refer to the oneor more subsequent stages as the primary compressor, although all stagesmay be part of one apparatus.

In some embodiments, the polymerization apparatus comprises, beside thepolymerization reactor, a high-pressure gas recycle line and alow-pressure gas recycle line for recycling unreacted monomers to thepolymerization process. The reaction mixture obtained in thepolymerization reactor may be transferred to a first separation vessel,frequently called a high-pressure product separator, and separated intoa gaseous fraction and a liquid fraction at a pressure of from 15 MPa to50 MPa. The gaseous fraction withdrawn from the first separation vesselis fed via the high-pressure gas recycle line to the suction side of thesecondary compressor. In the high-pressure gas recycle line, the gas maybe purified by several purifications steps from undesired componentssuch as entrained polymer or oligomers. The liquid fraction withdrawnfrom the first separation vessel, which in certain embodiments stillcomprises dissolved monomers such as ethylene and comonomers in anamount of 20-40% of weight, is transferred to a second separationvessel, frequently called low-pressure product separator, and furtherseparated, at reduced pressure, e.g. at an absolute pressure in therange of from 0.1 to 0.5 MPa, in polymeric and gaseous components. Thegaseous fraction withdrawn from the second separation vessel may be fedvia the low-pressure gas recycle line to the primary compressor, such asto the foremost of the stages. In some embodiments, the low-pressure gasrecycle line comprises several purifications steps for purifying the gasfrom undesired components.

In certain embodiments, the recycled gas coming from the low-pressuregas recycle line is compressed by the first stages of the primarycompressor to the pressure of the fresh feed of ethylenicallyunsaturated monomers, such as ethylene, and thereafter combined with thefresh gas feed and the combined gases to be compressed in the primarycompressor to the pressure of from 10 MPa to 50 MPa. In additionalembodiments, the primary compressor comprises five or six compressionstages, two or three before adding the fresh gas and two or three afteradding the fresh gas. In some embodiments, the secondary compressor mayhave two stages; a first stage, which compresses the gas from about 30MPa to about 120 MPa, and a second stage, which further compresses thegas from about 120 MPa to the final polymerization pressure.

Different configurations for suitable polymerization apparatuses suchas, for example, autoclave reactors are possible.

FIG. 1 shows schematically a typical set-up for a suitable tubularpolymerization reactor, without restricting the disclosure to theembodiments described therein.

As shown in FIG. 1, the fresh ethylene, which may be under a pressure of1.7 MPa, is compressed to a pressure of about 30 MPa by means of aprimary compressor (1) and then compressed to the reaction pressure ofabout 300 MPa using a secondary compressor (2). The molecular weightregulator is added to primary compressor (1). The reaction mixtureleaving the primary compressor (2) is fed to pre-heater (3), where thereaction mixture is preheated to the reaction start temperature of fromabout 120° C. to 220° C., and then conveyed to the tubular reactor (4).

The tubular reactor (4) is basically a long, thick-walled pipe withcooling jackets to remove the liberated heat of reaction from thereaction mixture by means of a coolant circuit (not shown). In someembodiments, the pipe is from about 0.5 km to 4 km, including from 1.5km to 3 km and from 2 km to 2.5 km long. In further embodiments, theinner diameter of the pipe is in the range of from about 30 mm to 120mm, including from 60 mm to 100 mm.

The tubular reactor (4) shown in FIG. 1 has four spatially separatedinitiator injection points (5 a), (5 b), (5 c) and (5 d) for feedinginitiators or initiator mixtures I1, I2, I3 and I4 to the reactor andfour reaction zones. By feeding suitable free-radical initiators, whichdecompose at the temperature of the reaction mixture, to the tubularreactor the polymerization reaction starts. The generated heat of thereaction increases the temperature of the reaction mixture, since moreheat is generated than can be removed through the walls of the tubularreactor. The rising temperature increases the rate of decomposition ofthe free-radical initiators and accelerates polymerization until allfree-radical initiators are consumed. Thereafter, no further heat isgenerated and the temperature decreases again since the temperature ofthe reactor walls is lower than the temperature of the reaction mixture.Accordingly, the part of the tubular reactor downstream of an initiatorinjection point, in which the temperature rises, is a reaction zone,while the part thereafter, in which the temperature decreases again, ispredominantly a cooling zone.

The amount and nature of added free-radical initiators determines howmuch the temperature rises and accordingly allows for adjusting thetemperature. In certain embodiments, the temperature rise in the firstreaction zone is set to be in the range of from 70° C. to 170° C., and50° C. to 130° C. for the subsequent reaction zones, depending on theproduct specifications and reactor configuration. The reaction mixtureleaves the tubular reactor (4) through a high-pressure let-down valve(6) and passes a post-reactor cooler (7). Thereafter, the resultingpolymer is separated off from any unreacted ethylene and other lowmolecular weight compounds (monomers, oligomers, polymers, additives,solvent, etc.) by means of a first separation vessel (8) and a secondseparation vessel (9), discharged and pelletized via an extruder andgranulator (10).

The ethylene and comonomers which have been separated off in the firstseparation vessel (8) are fed back to the inlet end of the tube reactor(4) in the high-pressure circuit (11) at 30 MPa. They are first freedfrom other constituents in at least one purification stage and thenadded to the monomer stream between primary compressor (1) and secondarycompressor (2). FIG. 1 shows one purification stage consisting of a heatexchanger (12) and a separator (13). It is however also possible to usea plurality of purification stages in certain embodiments. Thehigh-pressure circuit (11) may be used to separate waxes.

The ethylene which has been separated off in the second separationvessel (9), which further comprises, inter alia, the major part of thelow molecular weight products of the polymerization (oligomers) and thesolvent, is worked up in the low-pressure circuit (14), at an absolutepressure of from about 0.1 to 0.5 MPa, in a plurality of separators witha heat exchanger being located between each of the separators. FIG. 1shows two purification stages consisting of heat exchangers (15) and(17) and separators (16) and (18). It is however also possible to useonly one purification stage or more than two purification stages. Thelow-pressure circuit (14) may be used to separate oils and waxes.

Different configurations for suitable tubular polymerization reactor arealso possible. In some embodiments, the monomers are not only added atthe inlet of the reactor tube but also fed, possibly cooled, at aplurality of different points to the reactor. This feeding may be done,in certain embodiments, at the beginning of any further reaction zonesand especially if oxygen or air is used as an initiator, which isusually added to the monomer feed in the primary compressor.

According to some embodiments, the present disclosure relates toseparating polymeric and gaseous components of a polymer-monomer mixturewhere the polymeric components are obtained by high-pressurepolymerization of ethylenically unsaturated monomers in the presence offree-radical polymerization initiators. The polymer-monomer mixture ispreferably the liquid fraction withdrawn from the first separationvessel which usually operates at a pressure of from 15 MPa to 50 MPa anda temperature of from 120° C. to 300° C. The separation process of thepresent disclosure comprises the steps of introducing thepolymer-monomer mixture into a separation vessel; separating thepolymer-monomer mixture into a gaseous fraction and a liquid fraction;withdrawing the gaseous fraction from the top of the separation vessel;and withdrawing the liquid fraction from the bottom of the separationvessel. The separation is carried out at a pressure of from 0.12 MPa to0.6 MPa, such as from 0.15 MPa to 0.2 MPa, and a temperature of from120° C. to 300° C., including from 220° C. to 290° C. for ethylenehomopolymers and from 130° C. to 260° C. for ethylene copolymer. At theconditions of operating the separation vessel, the polymeric componentswithin the separating vessel are in a liquid state.

In certain embodiments, the separation vessel has a cylindrical shapewith a ratio of length to diameter L/D of from 0.6 to 10, such as from 2to 8, and is vertically arranged. The values for the vessel length andthe vessel diameter refer to the inner dimensions. In some embodiments,the lower end of the separation vessel is formed as a cone, where theheight of the cone is included in the vessel length. Preferably thetotal cone angle, i.e. the aperture of the cone, is in the range of from30° to 60°, more preferably in the range of from 40° to 50° andespecially in the range of from 43° to 47°. The separation vessel isequipped with an inlet pipe for introducing the polymer-monomer mixtureinto the separation vessel, where the inlet pipe extends vertically fromthe top into the separation vessel, and the inlet pipe is centrallyarranged in the separation vessel. The process for separating thepolymeric and gaseous components is further characterized in that theratio of the inner diameter of the inlet pipe at its lower end, i.e. atthe outlet of the pipe, and the inner diameter of the separating vesselin its cylindrical part is in the range of from 0.15 to 0.4, such asfrom 0.2 to 0.35. The realization of the required ratio of the innerdiameter of the inlet pipe at its lower end to the inner diameter of theseparating vessel in its cylindrical part may be achieved in someembodiments by utilizing, as inlet pipe, a tube which widens from top tobottom. The widening part of the inlet pipe may have a conical shapewith a total cone angle, i.e. an aperture, in the range of from 5° to40°, including from 10° to 30° and from 15° to 25°. Below the wideningpart, the inlet pipe can further have a cylindrical part with thediameter of the inlet pipe at its lower end. The inlet pipe can beremounted, i.e. pulled out of the separation vessel, by disconnecting aspecial manhole flange on top of the vessel.

According to an embodiment of the present disclosure, the inlet pipeextends for a distance into the separation vessel, which is from 25% to50% of the length of the separation vessel, including from 30% to 40% ofthe length of the separation vessel.

In further embodiments, the outlet for withdrawing the gaseous fractionfrom the separation vessel is located at the top of the separationvessel. Consequently, in some embodiments the distance from the lowerend of the inlet pipe to the outlet for withdrawing the gaseous fractionfrom the separation vessel is from 25% to 50% of the length of theseparation vessel, such as from 30% to 40% of the length of theseparation vessel.

The velocity of the polymer-monomer mixture at the lower end of theinlet pipe is, in some embodiments, in the range of from 0.3 m/s to 20m/s, such as from 0.5 m/s to 10 m/s and from 1 m/s to 5 m/s. Thevelocity of the polymer-monomer mixture at the lower end of the inletpipe can be determined from the mass-flow through the inlet pipe, theoverall density of the combination of gaseous and liquid components ofthe polymer-monomer mixture under the conditions, i.e. pressure andtemperature, within the separation vessel and the opening area of theinlet pipe at its lower end. The conduit providing the polymer-monomermixture to the separation vessel, which may be the pipe for conveyingthe polymer-monomer mixture from a first separation vessel to a secondseparation vessel of a high-pressure polymerization setup, may have abend in the range of from 60° to 120°, including from 80° to 100° andabout 90° C.

In some embodiments, the vertical velocity of the rising gas in theregion from the lower end of the inlet pipe to the outlet forwithdrawing the gaseous fraction from the separation vessel, whichoutlet is arranged above the lower end of the inlet pipe, e.g. at thetop of the vessel, is in the range of from 0.02 m/s to 2 m/s, from 0.04m/s to 1 m/s and from 0.05 m/s to 0.5 m/s.

In some embodiments, the gaseous fraction of the polymer-monomer mixturewithdrawn from the top of the separating vessel may be fed to the lowpressure recycle and, after purification and cooling, returned to thesuction side of the primary compressor. For purifying, the gas exitingthe separation vessel may first be fed to a heat exchanger in which thegas is cooled by hot water, and thereafter to a further separator, inwhich most of the carried over polymeric and oligomeric materials andimpurities are separated from the gas. By passing additional cooling andseparating steps, the gas may be further purified.

The liquid fraction of the polymer-monomer mixture withdrawn from thebottom of the separating vessel is, in some embodiments, transferred toa device for pelletizing the polymer, such as an extruder or acontinuous mixers, in which the material is usually further mixed,optionally degassed and then transferred into small solid particles,usually by passing the melt through a die and chopping the strand intoshort segments. Preferably the separation vessel is directly mounted tothe pelletizing device.

The level of the liquid phases in the separating vessel may be measuredby radiometric level measurement. Tthe filling level may be controlledby a product discharge valve such as a gate valve, a slide valve, a meltdiverter valve or a piston valve or by a gate valve. In an alternativeembodiment, the valve is customarily fully opened and the filling levelis controlled via the rotational speed of the pelletizing device such asthe extruder.

FIG. 2 shows schematically a cross-section of a separation vesselsuitable for the separating process of the present disclosure.

The liquid material withdrawn from separation vessel (8) (not shown inFIG. 2) is transferred to separation vessel (9) via a conduit (20),which has a 90° bend above the separation vessel (9). The liquidmaterial then enters separation vessel (9) from the top through a piping(21) which transforms in an inlet pipe (22). Inlet pipe (22) widens fromtop to bottom by a conical part (23) having a total cone angle α of 20°followed by a cylindrical part (24) of an inner diameter which is theinner diameter at the lower end of the inlet pipe.

At the bottom, separation vessel (9) tapers by a conical part (25)having a total cone angle β of 45°. The internal space of the separationvessel (26) is partly filled with a liquid fraction which is primarilymolten polymer. The liquid fraction exits the separation vessel at thebottom through piping (27). The gaseous fraction leaves the separationvessel at the top. FIG. 2 shows two outlets (28) and (29) for thegaseous fraction, it is however also possible to design the separationvessel (9) with one or three, or four or more outlets for the gaseousfraction.

At the top, the separation vessel is closed by a removable cover (30),to which the inlet pipe is removably connected via flange (31).

In some embodiments, the cylindrical part of the separation vessel isefficiently heated by means of coils or a jacket or heating panels,through which high- or medium-pressure saturated steam or pressurizedwater at a temperature of from 120 to 300° C. is passed, and the cone ismore intensively heated by means of coils or a jacket or heating panels,through which high- or medium-pressure saturated steam or pressurizedwater at temperature of from 120 to 300° C. is passed.

In one embodiment of the present disclosure, the surfaces within theseparation vessel, which are in contact with the liquid fraction, havean average roughness Ra of from 0.05 μm to 5 μm, such as from 0.5 μm to3.2 μm and from 0.1 μm to 1 μm. The low roughness may be achieved bygrinding or polishing of the surface, or by electro-polishing of thesurface. In an alternative embodiment, the surfaces within theseparation vessel which are in contact with the liquid fraction areprovided with a polytetrafluoroethylene coating. Thepolytetrafluoroethylene coating may be provided to the conical part ofthe separation vessel, the cyclindical part of the separation vessel andto the outer surface of the inlet pipe. As a consequence of theresulting low adhesion of the liquid fraction to the surfaces, theseparation vessel shows no fouling. This step results in an improvedpolymer quality with respect to gels and long operation periods of theseparation vessel, in which no inner cleaning may be required forseveral years.

Typical volumes for separation vessels for the separating processaccording to the present disclosure are, depending on plant capacity anddedicated products, in a range from 4 m³ to 100 m³ m for high-pressurepolymerization plants with an annual capacity of 80,000 to 500,000 tonsof LDPE.

In one embodiment of the present disclosure, the filling level of theliquid fraction in the separation vessel is measured by radiometriclevel measurements and is controlled by a product discharge valve whichoperates based on data coming from the level measurement. The fillinglevel may be kept in a range from a pre-defined minimum filling level toa pre-defined maximum level. In some embodiments, the distance from themaximum filling level to the lower end of the inlet pipe is from 20% to40% of the length of the separation vessel, including from 25% to 35% ofthe length of the separation vessel.

The level of the liquid fraction in the separation vessel may bemaintained as low as reasonably practical to minimize the probabilitythat polymeric material polymer is carried over from the separationvessel to the high-pressure recycle gas system, and to minimize theresidence time of the polymer in the separation vessel in order toreduce the risk of gel formation. Accordingly, the separation vessel ispreferably operated with a filling level which corresponds to an amountof polymer contained in the separation vessel which can be produced,running the high-pressure reactor with standard throughput, within 5min. It is further preferred that the maximum filling level correspondsto an amount of polymer contained in the separation vessel which can beproduced, running the high-pressure reactor with standard throughput,within 20 min.

The separating process according to the present disclosure brings abouthigh separation efficiency. The carry-over of polymeric material withthe gaseous fraction to the high-pressure recycle gas system is verylow. The carry-over in a high-pressure polymerization plant with anannual capacity of 300,000 tons LDPE may be 30 kg to 60 kg of polymericmaterial in 24 hours. The process also results in low fouling of thewalls of the separation vessel. The separation process according to thepresent disclosure gives a relatively plain surface level of the liquidfraction within the separation vessel, which facilitates an accuratemeasurement of the filling level. This plain surface level furtherallows for emptying of the separation vessel to a high extent bylowering the filling level as necessary, for example in the context of agrade change.

The process for separating polymeric and gaseous components of apolymer-monomer mixture according to the present disclosure may beutilized as part of a process for preparing ethylene homopolymers orcopolymers.

Accordingly, the present disclosure also refers to a process forpreparing ethylene homopolymers or copolymers from ethylenicallyunsaturated monomers in the presence of free-radical polymerizationinitiators at temperatures from 100° C. to 350° C. and pressures in therange of from 110 MPa to 500 MPa in a polymerization reactor comprisingsuch a process for separating polymeric and gaseous component. Thepolymerization may be carried out in one or more tubular reactors orautoclave reactors or combinations of such reactors.

While multiple embodiments are disclosed, still other embodiments willbecome apparent to those skilled in the art from the following detaileddescription. As will be apparent, certain embodiments, as disclosedherein, are capable of modifications in various obvious aspects, allwithout departing from the spirit and scope of the claims as presentedherein. Accordingly, the drawings and detailed description are to beregarded as illustrative in nature and not restrictive.

1. A process for separating a polymer-monomer mixture comprisingpolymeric components and gaseous components into a liquid fraction and agaseous fraction, the process comprising the steps of: (i) introducingthe polymer-monomer mixture into a separation vessel; (ii) separatingthe polymer-monomer mixture at a pressure of from 0.12 MPa to 0.6 MPaand a temperature of from 120° C. to 300° C. into a gaseous fraction anda liquid fraction; (iii) withdrawing the gaseous fraction from the topof the separation vessel; and (iv) withdrawing the liquid fraction fromthe bottom of the separation vessel, wherein the polymeric components ofthe mixture are obtained by high-pressure polymerization ofethylenically unsaturated monomers in the presence of at least onefree-radical polymerization initiator, and wherein a) the separationvessel has a vertically arranged cylindrical shape with a ratio oflength to diameter L/D of from 0.6 to 10; b) the separation vessel isequipped with an inlet pipe capable of introducing the polymer-monomermixture into the separation vessel; c) the inlet pipe extends verticallyfrom the top of the separation vessel into the separation vessel; and d)the ratio of the inner diameter of the inlet pipe at its lower end andthe inner diameter of the separating vessel in its cylindrical part isin the range of from 0.15 to 0.4.
 2. The process according to claim 1,wherein the inlet pipe is centrally arranged in the separation vessel.3. The process according to claim 1, wherein the polymer-monomer mixturehas been obtained as liquid fraction by separating a reaction mixtureobtained by high-pressure polymerization and the separation was carriedout at a pressure of from 15 MPa to 50 MPa and a temperature of from120° C. to 300° C.
 4. The process according to claim 1, wherein theinlet pipe extends for a distance into the separation vessel which isfrom 25% to 40% of the length of the separation vessel.
 5. The processaccording to claim 1, wherein the velocity of the polymer-monomermixture at the lower end of the inlet pipe is in the range of from 0.3m/s to 20 m/s.
 6. The process according to claim 1, wherein the verticalvelocity of the rising gas in the region from the lower end of the inletpipe to the outlet for withdrawing the gaseous fraction from theseparation vessel is in the range of from 0.02 m/s to 2.0 m/s.
 7. Theprocess according to claim 1, wherein the surfaces within the separationvessel which are in contact with the liquid fraction have an averageroughness Ra of at the most 3.2 μm.
 8. The process according to claim 1,wherein the surfaces within the separation vessel which are in contactwith the liquid fraction are provided with a polytetrafluoroethylenecoating.
 9. The process according to claim 1, wherein the angle of theconical part of the separator is in the range of from 30 to 60°.
 10. Theprocess according to claim 1, wherein inlet pipe has a conical partwhich has an angle in the range of from 5 to 25°.
 11. The processaccording to claim 1, wherein the level of the liquid fraction in theseparation vessel is measured by radiometric level measurement and iscontrolled by a product discharge valve.
 12. The process according toclaim 1, wherein the conduit providing the polymer-monomer mixture tothe separation vessel has a bend in the range of from 60° to 120°. 13.The process according to claim 1, wherein the separation vessel ismounted to a pelletizing device.
 14. A process for preparing ethylenehomopolymers or copolymers comprising polymerizing ethylenicallyunsaturated monomers in the presence of free-radical polymerizationinitiators at temperatures from 100° C. to 350° C. and pressures in therange of from 110 MPa to 500 MPa in a polymerization reactor, theprocess comprising a process for separating an obtained polymer-monomermixture comprising polymeric components and gaseous components into aliquid fraction and a gaseous fraction, the process comprising the stepsof: (v) introducing the polymer-monomer mixture into a separationvessel; (vi) separating the polymer-monomer mixture at a pressure offrom 0.12 MPa to 0.6 MPa and a temperature of from 120° C. to 300° C.into a gaseous fraction and a liquid fraction; (vii) withdrawing thegaseous fraction from the top of the separation vessel; and (viii)withdrawing the liquid fraction from the bottom of the separationvessel, wherein the polymeric components of the mixture are obtained byhigh-pressure polymerization of ethylenically unsaturated monomers inthe presence of at least one free-radical polymerization initiator, andwherein a) the separation vessel has a vertically arranged cylindricalshape with a ratio of length to diameter L/D of from 0.6 to 10; b) theseparation vessel is equipped with an inlet pipe capable of introducingthe polymer-monomer mixture into the separation vessel; c) the inletpipe extends vertically from the top of the separation vessel into theseparation vessel; and d) the ratio of the inner diameter of the inletpipe at its lower end and the inner diameter of the separating vessel inits cylindrical part is in the range of from 0.15 to 0.4.
 15. Theprocess according to claim 14, wherein the polymerization is carried outin one or more tubular reactors or autoclave reactors or combinations ofsuch reactors.
 16. The process according to claim 14, wherein the inletpipe is centrally arranged in the separation vessel.
 17. The processaccording to claim 14, wherein the separation is carried out at apressure of from 15 MPa to 50 MPa and a temperature of from 120° C. to300° C.
 18. The process according to claim 14, wherein the inlet pipeextends for a distance into the separation vessel which is from 25% to40% of the length of the separation vessel.
 19. The process according toclaim 14, wherein the velocity of the polymer-monomer mixture at thelower end of the inlet pipe is in the range of from 0.3 m/s to 20 m/s.20. The process according to claim 14, wherein the surfaces within theseparation vessel which are in contact with the liquid fraction have anaverage roughness Ra of at the most 3.2 μm.